A number of industrial and non-industrial applications use multi-component power generating devices. Industrial sites such as power plants may include a boiler-turbine unit in which a fuel-burning boiler generates thermal energy such as steam to operate one or more steam turbines, which in turns generates electricity. In these systems, one control objective is to adjust the power output to meet demands while maintaining stream pressure and temperature within desired ranges.
A typical steam generating system used in a power plant includes a boiler having a superheater section (having one or more sub-sections) in which steam is produced and is then provided to and used within a first, typically high pressure, steam turbine. To increase the efficiency of the system, the steam exiting this first steam turbine may then be reheated in a reheater section of the boiler, which may include one or more subsections, and the reheated steam is then provided to a second, typically lower pressure steam turbine. Both the furnace/boiler section of the power system as well as the turbine section of the power system must be controlled in a coordinated manner to produce a desired amount of power.
Moreover, the steam turbines of a power plant are typically run at different operating levels at different times to produce different amounts of electricity or power based on variable energy or load demands provided to the power plant. For example, in many cases, a power plant is tied into an electrical power transmission and distribution network, oftentimes referred to as a power grid, and the power plant provides a designated amount of power to the power grid. In this case, a power grid manager or control authority typically manages the power grid to keep the voltage levels on the power grid at constant or near-constant levels (that is, within rated levels) and to provide a consistent supply of power based on the current demand for electricity (power) placed on the power grid by power consumers. The grid manager may typically plan for heavier use and thus greater power requirements during certain times of the days than others, and during certain days of the week and year than others, and may run one or more optimization routines to determine the optimal amount and type of power that needs to be generated at any particular time by the various power plants connected to the grid to meet the current or expected overall power demands on the power grid.
As part of this process, the grid manager typically sends power demand requirements (also called load demand set points) to each of the power plants supplying power to the power grid, wherein the power demand requirements or load demand set points specify the amount of power that each particular power plant is to provide onto the power grid at any particular time. To effect proper control of the power grid, the grid manager may send new load demand set points for the different power plants connected to the power grid at any time, to account for expected and/or unexpected changes in power being supplied to or consumed from the power grid. For example, the grid manager may modify the load demand set point for a particular power plant in response to expected or unexpected changes in the demand (which is typically higher during normal business hours and on weekdays, than at night and on weekends) Likewise, the grid manager may change the load demand set point for a particular power plant in response to an unexpected or expected reduction in the supply of power on the grid, such as that caused by one or more power units at a particular power plant failing unexpectedly or being brought off-line for normal or scheduled maintenance.
In any event, while the grid manager may provide or change the load demand set points for particular power plants at any time, the power plants themselves cannot generally increase or decrease the amount of power being supplied to the power grid instantaneously, because power generation equipment typically exhibits a significant lag in response time due to the physical characteristics of these systems. For example, to increase the power output of a steam turbine based power generation system, it is necessary to change the amount of fuel being spent within the system, to thereby increase the steam pressure or temperature of the water within the boiler of the system, all of which takes a finite and non-trivial amount of time. Thus, generally speaking, power plants can only ramp up or ramp down the amount of power being supplied to the grid at a particular rate, which is based on the specifics of the power generating equipment within the plant.
In turbine based power plant control systems needing multiple control loops, standard multi-loop, single-input-single-output (SISO) strategies include turbine-following and boiler-following configurations. In turbine-following approaches, a power output is controlled by the boiler fuel input, and conversely, in boiler-following approaches, the power output is controlled by a steam throttle valve position, as the power output is directly proportional to the amount of steam supplied to the turbine. Generally, the turbine-following approach provides good control in the form of minimal variations of steam temperature and pressure, but the turbine-following approach cannot track the load demand quickly due to the slow steam generation process implemented in conventional power plants. In contrast, by controlling the throttle valve in the boiler-following approach, different amounts of steam may be immediately supplied to the turbine, but control is provided at the expense of depleting stored energy in the boiler, leading to main-steam pressure variations. Accordingly, for conventional plants operating at base-load, turbine-following approaches are preferred, while for conventional plants operating in ramp-load modes, the boiler-following approach is preferred.
Moreover, in power plants that use a boiler to generate power, a power plant controller typically uses a feedback controller to change a variable (commonly referred to as “trimming action”) to achieve a desired result based on information from system measurements to account for unknown system disturbances and process uncertainties. The power plant controller may also incorporate a feedforward (or anticipative) controller which foresees (predicts) future changes and provides quick action to increase or decrease the output power in response to an expected change in a load demand profile, which may be made either locally or by a remote dispatch (e.g., by the grid manager).
In current approaches, feedforward design is based on load demand profiles and is sometimes coupled with a dynamic “kicking” action which increases the response rate of the boiler as compared to a linear function of the load demand index. Feedback control often uses proportional-integral-derivative (PID) controllers.
An immediate drawback of using current feedforward approaches occurs due to the use of a steady-state load demand curve that does not provide guaranteed dynamic accuracy. Further, when the load target changes in the middle of a load ramping process, the current state is not taken into account in subsequent calculations. In other words, the conventional feedforward calculation treats the new load ramping process as if it always starts from a steady-state condition, and does not account for the present operational state of the equipment, which may include a current variable as well as a rate of change in that variable.